Is this a desktop or a laptop, and I'm guessing your refering to use with an igp.
With laptops and their BRIGHT idea to lock and limit what you can change it will normall be found in the Bios, if available.
Testing the HD3000
"so for these tests with allocated a 1GB UMA frame buffer size, and also tried three difference memory frequency/timing settings."
Did a google on UMA frame buffer. found this:
http://www.techarp.com/showfreebog.aspx?lang=0&bogno=245
In refernece to a dedicated GPU found this:
The UMA frame buffer is, basically, the amount of RAM which is allocated to the graphics card. Your graphics card has around 1GB of dedicated memory, which is more than enough. You will not see a performance increase by having a large UMA frame buffer, and, because RAM is much slower than dedicated memory, it may even cause a drop in performance. In this case, you should leave the UMA frame buffer as it is (and, depending on the size, consider reducing it).
Bottom line, I think it is bios controlled.
Added: In Bios Under Advance - Northbridge found IGP DVMT Memory my options were 256 MB and maxium (What ever max is LOL)